test: the default test should be more specific to shiny/golem instead of just expecting a body

## Description 

- Currently the test is the following one : 

```
import { test, expect } from '@playwright/test';

test('has body', async ({ page }) => {
  await page.goto('http://127.0.0.1:3000');
  await expect(page.locator('body')).toBeVisible();
});

```

I feel like we could have a better test that fits shiny / golem and that would serve as a good starting point